1. Field of the Invention
This invention relates to heat treatment processes for aluminum-lithium alloys. More specifically, the invention is a heat treatment process for aluminum-lithium alloys that will improve the alloys' formability into curved shapes.
2. Description of the Related Art
Large rocket domes are typically fabricated using aluminum-copper (Al—Cu) based alloys such as the Al—Cu alloy 2219. In order to be acceptable for rocket fuel tank applications, the aluminum alloy must possess requisite characteristics of strength, fracture toughness, and corrosion resistance. When used for a rocket dome, the aluminum alloy must also possess good formability to achieve the needed shape.
A lower overall rocket weight could be achieved through the use of aluminum-lithium based alloys as the material for rocket fuel tank domes because aluminum-lithium alloys have lower density and higher strength than the Al—Cu alloy 2219. However, aluminum-lithium alloys have rarely been used to fabricate rocket fuel tank domes because of the alloy's inherent low formability that make them susceptible to cracking during the forming operations.
Large rocket fuel tank domes can be manufactured using stretch forming or spin forming methods. Manufacturing a dome by stretch forming typically requires eight gore panels, or pie shaped pieces, 10 welding steps, and multiple operations and inspections to assemble these pieces into a full-scale fuel tank dome. Each gore panel requires stretch forming at room temperature to a large degree of forming. However, complex tensile and bending stresses induced during the stretch forming operations combined with the inherent low stretch formability of aluminum-lithium alloys cause high failure rates.
A large rocket fuel tank dome can also be manufactured by spin forming. To spin form a large rocket dome whose typical diameter is over 18 feet, the spin blank must be prepared by joining two smaller plates together using the friction stir welding (FSW) method. This is because the size of the aluminum-lithium alloy blank needed for spin forming is larger than commercially-available blanks. The welded blank is then spun formed to create a single-piece dome.
Certain technical challenges must be overcome when spin forming an aluminum-lithium alloy blank that includes FSW joint(s). The FSW process produces three zones that have distinct metallurgical structures inside and around the friction stir weld. The zone that is affected by the most severe plastic deformation is called the “nugget”. The second zone on each side of the nugget is the “thermo-mechanically affected zone”, which deforms to a lesser extent. The third zone is called the “heat affected zone” and is formed by the heat generated during FSW.
The as-welded blank is generally not suitable for spin forming because there is an unequal hardness distribution in the weld nugget, the thermo-mechanically affected zone, the heat affected zone, and the base material that is outside of these zones. This leads to an inhomogeneous forming behavior and uneven material thinning between the nugget and the base material. Therefore, the as-welded blank must be heat treated prior to spin forming.
To spin form an aluminum-lithium alloy blank joined by FSW, the hardness distribution across the spin blank must be equalized by a heat treatment called “post weld anneal”. More importantly, the formability for such blanks should not be adversely affected by this post weld annealing treatment.
Conventional post weld anneal processes typically use a simple “heat and hold” process that heats the blank to some elevated temperature and holds it there for some period of time before cooling. However, conventional post weld anneal processes applied to aluminum-lithium alloys have proven to be incapable of equalizing the hardness of a friction stir welded blank and obtaining the needed formability for spin forming. More specifically, after the conventional post weld anneal, the nugget zone always exhibits higher hardness than the other zones. Consequently, the weld nugget zone is very susceptible to cracking during spin forming. Thus, the low formability issues associated with lightweight aluminum-lithium alloys has limited the use of these alloys for large rocket fuel tank dome applications.